Brake unit consisting of a brake disk and a brake lining

ABSTRACT

A brake unit includes a brake disk made of a fiber-reinforced ceramic C/SiC composite and a brake lining tribologically interacting with this brake disk. The brake lining comprises the same material as the brake disk, in which case, at least in the area of the brake lining close to the surface, the hardness is lower than that of the brake disk. The brake unit exhibits a high stability to temperature while the wear is low and the service life is acceptable.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German Patent Document No. 19727 586.9, filed Jun. 28, 1997, the disclosure of which is expresslyincorporated by reference herein.

The present invention relates to a brake unit comprising a brake diskmade of a fiber-reinforced ceramic C/SiC composite, which can beproduced from a C/C pre-body, and a brake lining that tribologicallyinteracts with the ceramic C/SiC composite.

Brake disks of the above-mentioned type are known, for example, fromGerman Patent Document DE 44 38 455. Another fiber-reinforced ceramiccomposite is described in German Patent Application 197 11 829.1, whichis a prior publication.

A fiber-reinforced ceramic C/SiC composite has high temperaturestability and can therefore be very highly thermally stressed. Ittherefore presents a problem to find a suitable brake lining that canalso be thermally stressed and is sufficiently resistant to wear.Because of the high temperatures occurring in the contact area, the useof conventional brake linings with an organic binder system or ofconventional inorganically bound brake linings is particularlydifficult.

It is therefore an object of the present invention to provide a brakeunit of the above-mentioned type that has a sufficient service life andwherein the braking action is acceptable.

According to the present invention, the brake lining is made of afiber-reinforced ceramic C/SiC composite and can be produced from a C/Cpre-body, whose density, before silicating, at least in the area closeto the surface, is higher than the density of the C/C pre-body of thebrake disk.

The material of the brake lining is therefore basically the samematerial of which the brake disk is made. The production of the materialis known to a person skilled in the art, for example, from German PatentDocument DE 197 11 829.1. First, carbon fibers are mixed with a carbonprecursor, for example, pyrolizable synthetic materials and optionallyadditional substances and are pressed to form a pre-body, the so-called"green compact". The green compact is subjected to a pyrolysis, in whichthe synthetic materials change to pyrolysis carbon. This results in aporous C/C pre-body. The C/C pre-body is finally infiltrated with liquidsilicon and is heat-treated. This results in a ceramic,carbon-fiber-reinforced C/SiC body.

The larger and/or the more numerous the pores, the lower the density ofthis pre-body. Vice-versa, the smaller the pores and/or the lower theirnumber, the higher the density of the pre-body. Consequently, if thefraction of silicon or silicon carbide in the C/SiC body will be larger,the more porous the C/C pre-body; that is, the lower its density hadbeen. The silicon fraction and its distribution can therefore beinfluenced and controlled by way of the density of the pre-body.

The C/SiC brake disk is therefore paired with a brake lining which is ofthe same type of material, but is less hard. The lower hardness is theresult of the lower silicon or silicon carbide fraction, which is lowerthan in the brake disk. The ceramic fraction is therefore lower and thecarbon fraction is higher. The brake lining is therefore softer than thebrake disk. It surprisingly exhibits a considerably longer service lifeand lower rates of wear than a lining of the same type that hasapproximately the same hardness.

The lower hardness of the lining is therefore achieved by a higherdensity of the C/C pre-body. This density, in turn, is generated by alower pore volume of the C/C pre-body which is to be infiltrated withliquid silicon. Because of the low open pore fraction, the siliconcarbide fraction in the ceramized brake lining is relatively small andfinely distributed, and the carbon fraction is relatively high. Lowporosity results in fewer and finer pore channels into which the siliconcan penetrate.

The resulting ceramic material exhibits a behavior that is similar tothat of the C/C material; it is softer than the ceramic composite. Thecarbon causes the hardness, which is lower in comparison to completelyceramic silicon carbide bodies, and the mechanical adaptability of thebrake lining with respect to the micro form of the irregularities of thebrake disk surface. The remaining silicon carbide content causes ahigher resistance to wear and thermal stressability of the brake lining.

In contrast, in the case of the brake disks, a C/C pre-body is providedthat has a lower density and a higher porosity in comparison to thepre-body of the brake lining. The liquid silicon can enter through manysmall pores so that finally the silicon fraction will be high and theresulting ceramic material will have a comparatively high density.

The brake lining therefore consists of a fiber-reinforced ceramicmaterial that has a tribologically optimized behavior which is adjustedto the brake disk in a targeted manner. It exhibits low wear, a highservice life, a high damage tolerance and mainly a surprisingly highcoefficient of friction. The high coefficient of friction has the resultthat the pressure to be exercised during a braking maneuver can bereduced. The wear is therefore reduced again. In addition, it ispossible to make the brake power assist units smaller. This saves weightwhich, in turn, can lower the fuel consumption.

It is particularly advantageous to achieve a microscopically homogeneousmaterial structure with uniform characteristics.

With respect to its characteristics, the brake unit according to thepresent invention can be individually adapted to the respectiverequirements. If it is found, for example, that the C/SiC brake disk hasa wear that is too high for a special application, the fraction ofsilicon carbide in the disk can be lowered. Inversely, in the pertaininglining, the silicon carbide fraction should then be increased in orderto ensure an optimal braking action, a high service life and a low wear.The hardness of the brake disk and the hardness of the brake lining cantherefore be optimally adjusted to one another so that the whole brakeunit will have a high service life while the wear is low. "Optimally"means, for example, that the brake disk has a wear that can hardly bemeasured and the brake linings exhibit a wear behavior that can becompared with that of conventional brake linings.

Advantageously, the porosity of the C/C pre-body of the brake lining isat least in the area close to the surface approximately 20 to 30% lowerthan the porosity of the C/C pre-body of the brake disk. A preferredvalue for the density of the pre-body of the brake lining in the areaclose to the surface amounts to approximately 1.2-1.5 g/cm³, preferably1.3 g/cm³.

Advantageously, the brake lining and/or the brake disk consist of amaterial whose fibers are essentially isotropically oriented. As aresult, a uniformly high thermal conductivity can be achievedtransversely to the braking surface of the brake disk and the brakelining. This results in a lowering of the surface temperature in thecase of stress. Excessive surface temperatures may lead to an excessiveheating of the brake fluid.

Since disturbing noises can occur during a braking maneuver by means ofceramic brake disks and ceramic brake linings, it is advantageous toinclude so-called comfort stabilizers, such as Cu, CaF₂, MoS₂, and SbS₃.

With respect to the process according to the present invention, it isprovided that, for the manufacturing of a brake lining made offiber-reinforced C--SiC ceramic material, a carbon fiber body isproduced which has a specific pore and/or capillary volume; this carbonfiber body is infiltrated by carbon and/or a carbon precursor; and bypyrolysis, a porous C/C pre-body is produced which, in turn, isinfiltrated by liquid silicon, in which case the carbon is ceramized tosilicon carbide at least in the area of the pores and capillaries closeto the surface. The open pore and capillary volume of the C/C pre-bodybefore the liquid silicating should be adjusted to maximallyapproximately 60% by volume, preferably approximately 40 to 50% byvolume.

The adjustment is essentially influenced by the temperature during thepressing of the green compact, the applied pressing pressure and theused raw materials. The higher the pressing pressure, the higher thedensity of the resulting green compact, the lower the porosity of theC/C pre-body and the softer the resulting ceramic body. Anotherpossibility of adjusting the capillary volume consists of entering thecarbon precursor by means of resin transfer molding; that is, to actupon it by means of a certain pressure, charge it into the fiber body;and simultaneously evacuate the pores and capillaries of the carbonfiber body. A defined capillary size is reached according to theadjustment of the pressure or of the vacuum. The thus obtained greencompact is then pyrolized.

If no liquid carbon precursor is selected but a solid binding agent, theadjustment can take place by means of the type of the mixing of theconstituents during the manufacturing of the homogenate to be pressed.In particular, a powder or a paste of carbon particles can be doctoredinto the pores and capillaries. As the result, only areas close to thesurface are changed into pyrolysis carbon and silicated in the nextprocess step. The untreated surface areas and the interior remain as aC/C body.

Not only carbon powders, suspensions or pastes of carbon powders,graphite or pitch are suitable for being used as the carbon precursor,but also pyrolyzable polymers, such as vinyl resins orsilicon-containing polymers, for example, from the group of polysilanes,polycarbosilanes, polyborosilazanes, polysilazanes, polycarbosilazanesand polyborocarbosilizanes.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a light-microscopical micrograph of a brake disk of a brakeunit according to the present invention;

FIG. 2 is a light-microscopical micrograph of a brake lining with a highC/C content and a low SiC content;

FIGS. 3a, b are microsections in various directions of a C/C material,which is optimized with respect to the lining, before the silicating,specifically in the pressing direction (FIG. 3a) and perpendicularly tothe pressing direction (FIG. 3b);

FIG. 4 is a view of results of braking tests by means of a brake unitaccording to the present invention;

FIG. 5 is a view of results of braking tests by means of a C--SiC brakedisk and a brake lining made of the same material;

FIG. 6 is a view of results of braking tests by means of a C--SiC brakedisk and a C/C brake lining.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the micrograph of a brake disk of approximately 35to 45% by volume fibers, approximately 40 to 50% by volume SiC andmaximally approximately 15% by volume silicon. The structure ishomogeneous; the fibers are arranged isotropically. The density amountsto approximately 2.25 g/cm³.

FIG. 2 is a view of a C--SiC brake lining with a low SiC content. Thedensity amounts to approximately 2.15 g/cm³. For the manufacturing, agreen compact made of carbon fibers, carbon precursor and additionalsubstances is pressed at pressures above 100 bar.

Another optimized brake lining is illustrated in FIGS. 3a and 3b. FIG.3a very clearly shows the relatively small channels through which only alittle silicon has entered. The fiber structure has been retained, andthe C-fraction is high. FIG. 3b shows that the fibers are essentiallyisotropically distributed. The density of the material amounts toapproximately 2.15 g/cm³. The pressure during the pressing amounted toapproximately 100 to 200 bar. In this case, the pore channels arepressed relatively tightly together.

FIGS. 4 to 6 show test stand measurements with different combinations ofmaterials of the brake disk and the brake lining. FIG. 4 shows theresults obtained by means of the lining illustrated in FIGS. 3a and 3b.In all measurements, a brake disk according to FIG. 1 was used, whichhas a diameter of 285 mm. The linings measured 80×65 mm and werethermally shrunk into metallic holding devices. The applied pressuresamounted to 10-30 bar, at points up to 50 bar. The speeds were at 60-200km/h. In the case of the brake unit according to the present invention,the measured coefficient of friction was above 0.5. The temperature inthe disk center was below 400° C. The measured values exhibited a verygood constancy as a function of the applied pressures and speeds. Theconstancy of the temperature was also very good. A fading could not beobserved. The measured overall wear after the conclusion of the completemeasuring program amounted to 0.05 mm in the case of the brake disk and1 g in the case of the brake lining.

FIG. 5 shows the same measurement by means of a C/SiC brake disk and aC/SiC brake lining. The coefficient of friction was at a very good 0.6.The temperature in the disk center did not rise above 300° C. However,the wear data were unsatisfactory. In the case of the brake disk, thewear was 0.25 mm; in the case of the brake lining, 1.0 g. The hard andbrittle brake lining causes a measurable abrasion on the brake disk.This is accompanied by a considerable loss of material in the case ofthe brake lining itself.

FIG. 6 shows another comparative measurement by means of a C/SiC brakedisk and a C/C brake lining. Here, the coefficient of friction clearlywas poorer. The brake disk exhibited no measurable wear. In the case ofthe brake lining, the loss of material amounted to 2.5 g. In this case,the brake lining is too soft in order to withstand the effect of thehard and brittle brake disk over time.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A brake unit, comprising:a brake disk made of afiber-reinforced ceramic C/SiC composite produced from a C/C pre-body,and a brake lining that tribologically interacts with the ceramic C/SiCcomposite, wherein the brake lining comprises a fiber-reinforced ceramicC/SiC composite produced from a C/C pre-body whose density beforesilicating is, at least in the area close to the surface, higher thanthe density of the C/C pre-body of the brake disk.
 2. The brake unitaccording to claim 1, wherein the porosity of the C/C pre-body for thebrake lining, at least in the area close to the surface, isapproximately 20 to 30% lower than the porosity of the C/C pre-body forthe brake disk.
 3. The brake unit according to claim 1, wherein thedensity of the C/C pre-body for the brake lining, at least in the areaclose to the surface, is approximately 1.2 to 1.5 g/cm³.
 4. The brakeunit according to claim 3, wherein the density of the C/C pre-body forthe brake lining is approximately 1.3 g/cm³.
 5. The brake unit accordingclaim 1, wherein at least one of the brake disk and the brake liningconsist of a fiber-reinforced ceramic C/SiC composite whose fibers areessentially isotropically oriented.
 6. The brake unit according to claim1, wherein a green compact used for producing at least one of the brakedisk and the brake lining has at least one of a microscopicallyhomogeneous material structure and finely distributed small pores andchannels.
 7. A brake lining comprising a fiber-reinforced ceramic C/SiCcomposite produced from a C/C pre-body whose density before silicatingis, at least in the area close to the surface, higher than a density ofa C/C pre-body of a fiber-reinforced ceramic C/SiC composite brake disk.